In the machining process of aluminium alloy wheels, the degree of automation becomes higher and higher, and the front finish turning of products has been brought into the scope of automatic production by people. Due to specificity of the products, the fronts of vehicles are not allowed to be touched by hands and other things after machining, and therefore, mechanical arms must be used for grabbing lower rims of the wheels in order to realize the automatic machining of the wheels. The mechanical arms are required for putting the lower end surface of the wheel at a fixed height during grabbing every time in order that when wheel types are changed every time, the operational programs of robots are not changed, and the mechanical arms can recognize the position of the valve hole in order to realize the measurement of the height of the end surface inside a machine tool after clamping. Based the above situations, the device can solve the problems very well.